The present invention relates to a liquid crystal display device of an active matrix type used for displaying image data and character data from OA equipment or the like, and to the structure of an n-channel thin-film transistor used for this device.
A thin-film transistor (hereinafter abbreviated as TFT) has heretofore been used for driving liquid crystals of a pixel in a panel of a liquid crystal display device of the directly-viewing type. A simple switching element suffices for the performance of the TFT and, hence, amorphous silicon has been used as a semiconductor thin film. On the other hand, a liquid crystal display device of the projection type requires a high degree of brightness, and the TFT must be realized in a small size to increase the transmission factor. However, it was not allowed to decrease the size of the TFT formed by amorphous silicon since its current driving ability was so small. Therefore, so-called high-temperature polysilicon has been developed featuring an increased current driving ability by using quartz glass as a substrate and polycrystallizing amorphous silicon at a temperature as high as 900° C. or higher.
However, quartz glass is very expensive and drives up the cost of production. Therefore, so-called low-temperature polysilicon has been developed by using an inexpensive glass substrate and polycrystallizing amorphous silicon by the irradiation with a laser beam.
In recent years, strikingly improved performance has been exhibited by the TFT formed by using the low-temperature polysilicon. Under such circumstances, it is becoming a tendency to utilize the TFT not only as a switching element for driving the pixels of a panel in a liquid crystal display device but also for the peripheral drive circuits in the liquid crystal display device. Moreover, a liquid crystal display device such as system-in-display is emerging being furnished with a memory function as well as various functions of CPU, interface, I/O and input by pen by using TFTs. In these cases, the role played by the TFT is not only limited to that of a simple switching element; i.e., performance and reliability are required by taking the logic circuits into account.
When the TFT is used as a logic element, eight kinds of voltage patterns will be applied to the three terminals of gate, source and drain as tabulated below, wherein “H” denotes a high level and “L” denotes a low level.
Patterns12345678electrodeGateHLHLHLLHSourceHHLLLHLHDrainLLHHLHLH
So far, the TFT has been used for driving a liquid crystal pixel, and the above-mentioned patterns 1 to 4 have been exclusively used, i.e., relations of potential difference across the source and the drain have been exclusively used. When a potential difference develops across the source and the drain, a high electric field is established in the TFT, and a carrier having abnormally high energy (hereinafter referred to as hot carrier) is generated. The hot carrier that is injected into the gate oxide film causes a problem of deterioration in the characteristics of TFT.
It has heretofore been attempted to solve the problem of hot carrier that generates when a high electric field is applied across the source and the drain. As a means for solving this problem, there have been proposed a lightly doped drain (LDD) structure and a double drain structure as disclosed in “Submicron Device 2”, by Mitsumasa Koyanagi, Maruzen Co., 1995, p. 187. According to these structures, a high electric field applied across the source and the drain is relaxed to prevent the generation of hot carrier. These structures are with the case when a single crystal is used as a semiconductor. The same, however, also holds true even in the case of TFT.